CONCENTRATED SPECTRALLY SEPARATED MULTICONVERTER PHOTOBOLTAIC SYSTEMS AND METHODS THEREOF
A solar conversion apparatus and method includes two or more conversion cells and a reflector assembly. Each of the two or more solar conversion cells is responsive to a different one of at least a first band of wavelengths from solar radiation and a second band of wavelengths from the solar radiation. The reflector assembly comprises at least two integrated reflective sections. One of the at least two reflective sections is positioned to reflect and direct the first band of wavelengths towards one of the two or more solar conversion cells and another one of the at least two reflective sections is positioned to reflect and direct the second band of wavelengths towards another one of the two or more solar conversion cells. At least one of the two integrated reflective structures further comprises a Fresnel microstructure.
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This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/165,129, filed Mar. 31, 2009, which is hereby incorporated by reference in its entirety.
FIELDThis invention generally relates to photovoltaic converters and, more particularly, to concentrated solar photovoltaic converters.
BACKGROUNDOptical concentrators are widely used in solar photovoltaic converters for two important reasons. First, they allow for reduced system cost because less photovoltaic conversion material is required which is by far the most expensive component in a photovoltaic system. Typically, concentrated photovoltaic systems have a photovoltaic cell that has less than 1% of the area of a photovoltaic cell used in an equivalent conversion apparatus. Second, it is well known that photovoltaic cells illuminated by higher flux densities achieve higher solar-to-electricity conversion efficiencies.
A typical prior art concentrating photovoltaic system 10 is illustrated in
A Fresnel optical element can be of two types: one that operates in transmission which is called a Fresnel lens; and one that operates in reflection which is called a Fresnel mirror or Fresnel reflector. Both Fresnel lenses and Fresnel reflectors are commonly employed in solar concentrators and both include a Fresnel microstructure with a series of rather shallow grooves that are generally sawtooth in cross-section. The longer surface of each groove that performs the optical work is called the slope surface and the shorter surface that connects the slope surfaces together is called the draft or riser surface. The angle of the slope surface generally changes slightly from groove to groove being more shallow near the center of the Fresnel and steeper at the edges. At the same time the depth of the draft or riser surface are smaller near the center of the Fresnel microstructure and greater at the edge.
There are two major problems with this prior art concentrating photovoltaic system 10. First, the focal point 5 is not a point because of chromatic aberration. Instead, the focal point can be several centimeters in diameter depending on the geometry of the optical configuration and the range of wavelengths passed by the Fresnel lens 2. An ideal condensing Fresnel lens would transmit and bring to a focus all optical energy within the wavelengths of the sun that contain significant amounts of energy. Typically, the wavelengths of the sun range from about 350 nm to about 1900 nm. The dispersive nature of the material comprising the condensing Fresnel lens 2 causes the refractive index of the material to vary significantly over this range, which in turn causes the optical power of the condensing Fresnel lens 2 to vary as a function of wavelength, which in turn causes the diameter of the focal spot 5 to also vary with wavelength. To compensate for this, additional condensing optics can be installed atop the photovoltaic cell 4 or the photovoltaic cell 4 can be made substantially larger to ensure that it captures all of the energy of the worst-case focal spot 5. Both of these solutions, however drive up system cost and complexity, and reduce efficiency.
A second problem with this prior art concentrating photovoltaic system 10 is that only one solar photovoltaic cell 4 is used for each condensing Fresnel lens 2. Utilizing tandem photovoltaic cells having a variety of stacked photovoltaic junction bandgaps can significantly improve photovoltaic conversion efficiency. These tandem photovoltaic cells are formed by growing two or three photovoltaic cells atop one another in a semiconductor foundry.
An example of a typical triple junction cell 6 is illustrated in
A solar conversion apparatus includes two or more conversion cells and a reflector assembly. Each of the two or more solar conversion cells is responsive to a different one of at least a first band of wavelengths from solar radiation and a second band of wavelengths from the solar radiation. The reflector assembly comprises at least two integrated reflective sections. One of the at least two reflective sections is positioned to reflect and direct the first band of wavelengths towards one of the two or more solar conversion cells and another one of the at least two reflective sections is positioned to reflect and direct the second band of wavelengths towards another one of the two or more solar conversion cells. At least one of the two integrated reflective structures comprises a Fresnel microstructure.
A method of making a solar conversion apparatus includes providing two or more solar conversion cells where each of the two or more solar conversion cells is responsive to a different one of at least a first band of wavelengths from solar radiation and a second band of wavelengths from the solar radiation. At least one of the two reflective sections is positioned to reflect and direct the first band of wavelengths towards one of the two or more solar conversion cells and another one of the at least two reflective sections is positioned to reflect and direct the second band of wavelengths towards another one of the two or more solar conversion cells. At least one of the two integrated reflective structures comprises a Fresnel microstructure.
This technology provides a number of advantages including providing a more efficient, better performing, and economical solar conversion apparatus. This technology is able to avoid prior problems with large focal spot sizes and the use of a large and expensive, multi junction photovoltaic cell by utilizing a lower reflector assembly comprising one or more Fresnel reflectors arranged in a cascade configuration. Each of these Fresnel reflectors is reflective to a selected band of wavelengths and is transmissive to other wavelengths that are in turn reflected by lower Fresnel reflectors. Additionally, each Fresnel reflector includes a microstructure that reflects and brings to a focus onto a photovoltaic cell a selected band of wavelengths that the photovoltaic cell is most responsive to. The resulting solar conversion apparatus has a high concentration ratio, is lossless over the range of wavelengths emitted by the sun that have significant energy content, and effectively directs the concentrated solar energy to the appropriate single or multi junction photovoltaic cell. Furthermore, since the semiconductor junctions are not fabricated into the tandem PV-cell but instead are separated into separate PV-cells, a junction producing less photocurrent than the other junctions will not restrict the output of the other junctions.
An exemplary solar conversion apparatus 20 is illustrated in
Referring more specifically to
Referring more specifically to
The Fresnel microstructure 68 has a series of triangular grooves having slope surfaces 66 and draft surfaces 64. The slope surfaces 66 which perform the work of optically bending the incident solar energy 22 are designed so the focal length of the condensing lens 30 is approximately twice the cavity depth D (shown in
Fresnel surfaces, whose optical axis is substantially collinear with the optical axis 1 of the concentrator.
Referring to
The layers 48C, 48B, and 49 will now be described with reference to
In this example, light of a wavelength greater than 600 nm is transmitted through the reflective filtering layer 49 and is incident on the reflective filtering layer 48B that has the spectral reflectance as shown in
Light of a wavelength greater than about 900 nm is transmitted through the reflective filtering layer 49 and the reflective filtering layer 48B and is incident on the reflective layer 48C that has the spectral reflectance as shown in
Referring back to
The two reflective layers 48B and 48C are internal to the reflector assembly 32. The reflective filtering layer 48B is installed onto the Fresnel microstructure 53 resulting in a Fresnel mirror that is reflective only to the band of wavelengths as described above in connection to
The reflective layer 48C is installed onto the Fresnel microstructure 50 resulting in a Fresnel mirror that is reflective only to the band of wavelengths as described above in connection to
Referring back to
The photovoltaic cells 36A-36C can be made from a wide variety photovoltaic cell materials and alloys. By way of example only, graphs in
The solar apparatus conversion system 20 economically and efficiently separates the solar energy into three discrete wavelength groupings and directs each group of concentrated solar energy onto the particular photovoltaic cells 36A-36C that is optimal for the wavelengths that are directed to it. As illustrated in
Referring back to
The operation of the solar conversion apparatus 20 will now be described with reference to
The condensing lens 30 causes any of the incident solar radiation 22 to converge. These converging rays, such as converging white light ray 24 (which contains all wavelengths of groupings λA, λB, and λC), are incident on the reflective filtering layer 49 of the reflector assembly 32. Due to the reflectance characteristics of the reflective filtering layer 49, light rays 26A of wavelength group λA are reflected in accordance with the Law of Reflection, and all other wavelength groups (λB and λC) are transmitted into the reflector assembly 32 in accordance with Snells Law. The prescription of the condensing lens 30 is such that the light rays 26A of wavelength group λA are brought to a focus on the photovoltaic cell 36A. If the location of the photovoltaic cell 36A is such that it is coplanar with the condensing lens 30, then the focal length of the condensing lens 30 must be approximately twice the distance between condensing lens 30 and the reflective filtering layer 49, which is 2×D. The photovoltaic cell 36A is selected to be highly responsive to wavelength group λA of incident rays 26A and converts the incident solar energy of these rays into electricity with very high efficiency.
After passing through reflective filtering layer 49 and refracting into the reflector assembly 32, wavelength group λB propagates through layer 205 and into layer 204 where it becomes incident on reflective filtering layer 48B. Reflective filtering layer 48B is installed onto the Fresnel microstructure 53 and therefore cooperatively forms a Fresnel mirror. Additionally, reflective filtering layer 48B in accordance with its spectral reflectance profile shown in
After passing through reflective filtering layer 49 and refracting into the reflector assembly 32, wavelength group λC propagates through layers 205, 204, 203 and into layer 202 whereupon it becomes incident on reflective layer 48C. Reflective filtering layer 48B is not reflective to wavelength group λC and these rays pass through reflective filtering layer 48B substantially undeviated in direction. Additionally, the reflective layer 48C is installed onto the Fresnel microstructure 50 and therefore cooperatively forms a Fresnel mirror. The reflective layer 48C in accordance with its spectral reflectance profile shown in
Accordingly, as illustrated and described herein, the solar conversion apparatus 20 offers a considerable performance and economic advantage over prior art single junction solar concentrators and triple junction tandem photovoltaic cells. Additionally, although the solar conversion apparatus 20 is illustrated with three photovoltaic cells 36A-36C, the solar conversion apparatus can have additional photovoltaic cells with improved conversion efficiency as illustrated in
An exemplary method for constructing the condensing Fresnel lens 30 will now be described with reference to
A Fresnel microstructure 68 is installed on the lower side of the condensing Fresnel lens 30. The microstructure 68 comprises a polymer material, such as a UV-cured resin, although other types of materials can be used, such as silicone which has transmittance over the entire 350 nm to 18900 nm solar insolation range and it is relatively immune to UV damage from the solar UV light. The prescription of the slope surfaces 66 of the Fresnel microstructure is formed so that it results in a focal length of the condensing Fresnel 30 of 2D for the shorter wavelength band (i.e., λA). Longer wavelengths will generally see a longer focal length because the refractive index of the material comprising the microstructure 68 is lower at the longer wavelengths because of the materials dispersion.
An exemplary method for constructing and assembling the reflector assembly 32 will now be described with reference to
Next, a specularly-reflecting reflective coating layer 48C is applied to the slope surfaces of the microstructure 50, resulting in the lower reflecting Fresnel 61C shown in
In addition to the lower reflecting Fresnel 61C, a reflective filtering Fresnel 61B also is prepared in a process similar or identical to the process described above for the lower reflecting Fresnel 61C. After both the lower reflecting Fresnel 61C and the middle reflective filtering Fresnel 61B are available, they must be bonded together. As shown in
The encapsulant adhesive 51 is allowed to cure, dry, or otherwise harden resulting in the assembly depicted in
Next, the reflector assembly portion 61A comprising a substrate layer 205 and reflective filtering layer 49 are prepared. Both the upper and lower sides of the substrate upper layer 205 are planar and the substrate upper layer 205 is made from polymer, although other types of materials can be used, such as glass. The reflective filtering layer 49 is an interference stack of thin films that reflects the desired band of wavelengths (i.e., λA). After both reflecting Fresnel assembly portions 61C and 61B and reflector assembly portion 61A are available, they must be bonded together. As shown in
The transparent encapsulant adhesive 54 is then allowed to cure, dry, or otherwise harden, resulting in the reflector assembly 32 depicted in
In other examples, the solar conversion apparatus assembly process can be streamlined if, instead of having two optically active devices (the condensing lens 30 and the reflector assembly 32), there were only one. This can be accomplished by dispensing with the condensing lens 30 and by installing an additional reflecting Fresnel mirror within the reflector assembly.
Referring to
Referring more specifically to
In operation Fresnel mirror 78A reflects and focuses its band of wavelengths (e.g., λA) onto photovoltaic cell 36A and transmits all others (e.g., λB and λC) substantially undeviated. Fresnel mirror 78B reflects and focuses its band of wavelengths (e.g., λB). onto photovoltaic cell 36B and transmits all others (e.g., λC) substantially undeviated. Fresnel mirror 78C reflects and focuses all remaining wavelengths (e.g., λC) onto photovoltaic cell 36C. With this configuration for the solar conversion apparatus 70, the size of the reflector assembly 72 must be increased to fill the entire rear bulkhead surface 34 due to the absence of a condensing lens 30.
One problem that is common to the embodiments described thus far has to do with the placement of the photovoltaic cells 36A-36C on the optical axis 1. This gives rise to the shadow-loss problem wherein a portion of the light that would be incident on an upper photovoltaic cell, such as photovoltaic cell 36A is blocked by a lower photovoltaic cell such as photovoltaic cell 36B. In other words photovoltaic cell 36A is partly shadowed by lower photovoltaic cell 36B. Accordingly, to overcome the shadow losses it is necessary to install the photovoltaic cells in an off-axis location outside the cone of converging rays.
The side-view of one such off-axis solar conversion apparatus 80 is shown in
The reflector assemblies 86-89 each comprise four reflective filtering Fresnels installed as described earlier that split and reflect the incident converging solar energy 84 into four groups of light 92A, 92B, 92C, and 92D (each containing only a limited band of wavelengths) and focus the four groups of light 92A-92D onto the four different photovoltaic cells 90A-90D, respectively that are most responsive to the wavelengths of light incident directed onto them.
The four different types of photovoltaic cells 90A, 90B, 90C, and 90D are mounted on the internal bulkhead 94. Additionally, the four photovoltaic cells 90A, 90B, 90C, and 90D are located away from the optical axis 1 and between the converging rays 84 so that there are no shadowing effects that reduce system efficiency. The four different types of photovoltaic cells 90A, 90B, 90C, and 90D and are selected to be responsive to four different wavelength bands of light spread across the solar energy spectrum from about 350 nm to about 1800 nm, although other numbers of photovoltaic cells responsive to other bands of wavelengths can be used. As illustrated in
With this solar conversion apparatus 80, the photovoltaic cells 90A, 90B, 90C, and 90D are located where the corners of several concentrators meet so one photovoltaic cell can collect light of its wavelength band from four different concentrators. As a result, the number of photovoltaic cells in solar conversion apparatus 80 has been reduced by 75%. This is particularly evident in the plan view shown in
In operation, the solar conversion apparatus 80 accepts solar radiation 22 that is incident on the condensing lens 82 which condenses the solar radiation into converging cones of light 84. The converging cones of light pass through apertures 96 in the internal bulkhead 94 and critically illuminates the reflector assemblies 86, 87, 88, and 89. The reflector assemblies 86, 87, 88, and 89 each comprise a different Fresnel microstructure which is used to reflect light towards the corresponding one of the photovoltaic cells 90A, 90B, 90C, and 90D responsive to the reflected band of wavelengths of the solar energy.
Referring to
Eliminating the upper condensing Fresnel lens from the solar conversion apparatus offers several advantages, including: 1) the cost of the condensing lens is eliminated; 2) the Fresnel reflection losses at the input and output surfaces are eliminated thereby increasing efficiency, and 3) the molds for the reflecting mirrors of the microstructure of the reflector assembly 173 are circularly symmetric and easier to tool and fabricate, thereby reducing the costs associated with the reflector assembly as compared to the solar conversion apparatus 80 shown in
A magnified view of a small section 177 of reflector assembly 173 is shown in
The encapsulating adhesive layers 192 and 193 are used to secure the layers in the same manner as described with earlier examples. The microstructures 194 and 195 in layers 183 and 181 go from side-to-side in this view, and are represented by dashed lines. The encapsulating adhesive, while also present in layers 181 and 183, are not explicitly shown from this view.
While four Fresnel mirrors and four types of photovoltaic cells are described as being used in solar conversion apparatus 170, a lower number, such as one, two, or three, can be used, or a higher number, such as six, can be used. Additionally, the photovoltaic cells can be single junction cells or multi junction type photovoltaic cells.
The operation of the solar conversion apparatus 170 is the same as the operation of the solar conversion apparatus 80, except that with the solar conversion apparatus 170 there is no condensing lens that accepts and condenses the solar radiation into converging cones of light. Instead, the solar radiation passes directly through to the microstructures 191, 190, 194, and 195 in layers 187, 185, 183, and 181 and is correspondingly reflected in bands to the laterally arranged photovoltaic cells with the appropriate responsivity to the reflected band of wavelengths.
Referring to
Accordingly, as illustrated and described herein this technology provides a number of advantages, including providing a more efficient, better performing, and economical solar conversion apparatus. This technology is able to avoid prior problems with large focal spot sizes and the use of a large and expensive, multi junction photovoltaic cell by utilizing a lower reflector assembly comprising one or more Fresnel reflectors arranged in a cascade configuration. Each of these Fresnel reflectors is reflective to a selected band of wavelengths and is transmissive to other wavelengths that are in turn reflected by lower Fresnel reflectors. Additionally, each Fresnel reflector includes a microstructure that reflects and brings to a focus onto a photovoltaic cell a selected band of wavelengths that the photovoltaic cell is most responsive to. The resulting solar conversion apparatus has a high concentration ratio, is lossless over the range of wavelengths emitted by the sun that have significant energy content, and effectively directs the concentrated solar energy to the appropriate single or multi junction photovoltaic cell.
Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Further, the recited order of elements, steps or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be explicitly specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.
Claims
1. A solar conversion apparatus comprising:
- two or more solar conversion cells, each of the two or more solar conversion cells is responsive to a different one of at least a first band of wavelengths from solar radiation and a second band of wavelengths from the solar radiation; and
- a reflector assembly comprising at least two integrated reflective sections, one of the at least two reflective sections is positioned to reflect and direct the first band of wavelengths towards one of the two or more solar conversion cells and another one of the at least two reflective sections is positioned to reflect and direct the second band of wavelengths towards another one of the two or more solar conversion cells and at least one of the two integrated reflective structures comprises a Fresnel microstructure.
2. The apparatus as set forth in claim 1 wherein the two or more solar conversion cells are physically separated from each other.
3. The apparatus as set forth in claim 1 wherein the two or more solar conversion cells are substantially aligned along an optical axis which extends through the reflector assembly.
4. The apparatus as set forth in claim 1 wherein the two or more solar conversion cells are offset from an optical axis which extends through the reflector assembly.
5. The apparatus as set forth in claim 4 wherein the two or more solar conversion cells are each substantially positioned along a plane through which the optical axis intersects.
6. The apparatus as set forth in claim 4 wherein the two or more solar conversion cells further comprise at least four of the solar conversion cells which are each responsive to a different one of at least the first band of wavelengths from solar radiation, the second band of wavelengths from the solar radiation, a third band of wavelengths from the solar radiation, and a fourth band of wavelengths from the solar radiation.
7. The apparatus as set forth in claim 1 wherein at least one of the two or more solar conversion cells comprises a single junction photovoltaic cell.
8. The apparatus as set forth in claim 1 further comprising at least one mounting assembly that supports the two or more solar conversion cells with respect to the reflector assembly.
9. The apparatus as set forth in claim 1 wherein the one of the at least two reflective sections that is at least partially reflective to the first band of wavelengths is substantially transmissive to at least the second band of wavelengths.
10. The apparatus as set forth in claim 9 wherein the at least one of the reflective sections with the Fresnel microstructure further comprises an internal reflector on a surface of the Fresnel microstructure, the internal reflector is at least partially reflective to one of the first band of wavelengths and the second band of wavelengths, the Fresnel microstructure is optically configured to direct the one of the first band of wavelengths and the second band of wavelengths at least partially reflected by the internal reflector towards one of the two or more solar conversion cells.
11. The apparatus as set forth in claim 10 wherein the internal reflector is at least partially transmissive to at least the other one of the first band of wavelengths and the second band of wavelengths.
12. The apparatus as set forth in claim 9 wherein the reflective system further comprises:
- a first spacer layer having one surface adjacent at least one of the reflective sections;
- an intake reflector which is at least partially transmissive to at least the first band of wavelengths and the second band of wavelengths on an opposing surface of the first spacer layer; and
- a second spacer layer between the at least two reflective sections.
13. The apparatus as set forth in claim 1 wherein the first band of wavelengths is substantially non-overlapping with the second band of wavelengths.
14. The apparatus as set forth in claim 1 further comprising at least one condensing lens apparatus positioned to converge the solar radiation on the reflector assembly.
15. The apparatus as set forth in claim 14 wherein the at least one condensing lens assembly further comprises a Fresnel microstructure optically configured to converge at least a portion of the solar radiation on the reflector assembly.
16. The apparatus as set forth in claim 15 further comprising at least a third one of the two or more solar conversion cells, the third one of the two or more solar conversion cells is responsive to a third band of wavelengths from solar radiation, wherein the Fresnel microstructure in the at least one condensing lens assembly is further optically configured to converge at least a portion of the third band of wavelengths on to the third one of the two or more solar conversion cells.
17. The apparatus as set forth in claim 16 wherein the first band of wavelengths, the second band of wavelengths and the third band of wavelength are substantially non-overlapping with respect to each other.
18. The apparatus as set forth in claim 1 wherein the one of the two or more solar conversion cells positioned closer to a first incident location for receiving the solar radiation is responsive to a shorter band of wavelengths than the other of the two or more solar conversion cells positioned further from the first incident location.
Type: Application
Filed: Mar 31, 2010
Publication Date: Jun 28, 2012
Applicant: REFLEXITE CORPORATION (AVON, CT)
Inventor: James F. Munro (Webster, NY)
Application Number: 13/260,879
International Classification: H01L 31/052 (20060101);